Breakup dynamics for droplet formation in a flow-focusing device: Rupture position of viscoelastic thread from matrix

The breakup dynamics of the viscoelastic dispersed thread for droplet formation in a flow-focusing device was investigated by using a high-speed digital camera. Four breakup regimes of the viscoelastic thread were observed: besides the thread rupturing at both ends or at the downstream end likewise in Newtonian fluid, the breakup could also occur at the upstream end, displaying two additional patterns. For each breakup type, the breakup of the viscoelastic thread could be divided into a "flow-driven" stage controlled by the inertia of the dispersed phase and the dynamical radial pressure of the continuous phase, and an "elastocapillary" stage controlled by the elastic and capillary forces. The "elastocapillary" stage includes the stretching process and the fluid drainage process, depending on whether the balance between the elastic forces and the capillary forces reaches or not. The differences in breakup dynamics and liquid-liquid interface shape for the four breakup types depend exclusively on the elasticity of the viscoelastic thread. (C) 2016 Elsevier Ltd. All rights reserved.